The present invention relates to a method for forming distinctive and decorative colored streak patterns inscribed or engraved in the surface of a shaped article of aluminum or an aluminum-based alloy (hereinafter referred to simply as aluminum) by an electrochemical means.
The inventors have previously proposed a method for forming decorative colored streak patterns on the surface of an aluminum shaped article (see Japanese Patent Kokai 56-69390). In this method, colored streak patterns are formed on the surface of an aluminum shaped article by the successive steps of carrying out electrolysis with alternating current with the aluminum shaped article having been provided with an anodized surface film as an electrode in an electrolyte solution containing at least one of the alkali metal salts and alkaline earth metal salts of organic or inorganic acids, carrying out electrolysis with direct current with the aluminum shaped article as the cathode in the same electrolyte solution as in the first electrolysis to form streak patterns engraved in the surface thereof, anodizing the surface of the aluminum shaped article thus provided with streak patterns in an electrolytic solution containing an inorganic acid and/or an organic acid to form an oxide film thereon and subjecting the oxide film to a coloring treatment.
In this prior art method, very distinctive streak patterns are obtained with clear difference in the color gradation between the streak-wise patterns and the background areas but the method is unsatisfactory in respect of the uniformity of the pattern form-up distribution of the streak patterns in unit surface area. This is because the aluminum shaped article to be finished to a final product usually has a complicated form so that the streak patterns are formed preferentially as a trend on the areas where the density of the electrolytic current is large such as the areas facing the counterelectrode with a smaller inter-electrode distance or the areas directly facing the counterelectrode.
Therefore, it has been an important problem in the aluminum industry to develop a method for forming decorative streak patterns on the surface of a shaped aluminum article with high uniformity of the pattern distribution over the whole surface of the article.